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Chapter 8
Configuring Spanning Tree Protocol
Figure 8-1
LAN segment A
LAN segment B
Spanning-Tree Interface States
Propagation delays can occur when protocol information passes through a wireless LAN. As a result,
topology changes can take place at different times and at different places in the network. When an
interface transitions directly from nonparticipation in the spanning-tree topology to the forwarding state,
it can create temporary data loops. Interfaces must wait for new topology information to propagate
through the LAN before starting to forward frames. They must allow the frame lifetime to expire for
forwarded frames that have used the old topology.
Each interface on a bridge using spanning tree exists in one of these states:
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An interface moves through these states:
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OL-4059-01
Spanning-Tree Topology
Bridge 1
Bridge 3
Blocking—The interface does not participate in frame forwarding.
Listening—The first transitional state after the blocking state when the spanning tree determines
that the interface should participate in frame forwarding.
Learning—The interface prepares to participate in frame forwarding.
Forwarding—The interface forwards frames.
Disabled—The interface is not participating in spanning tree because of a shutdown port, no link on
the port, or no spanning-tree instance running on the port.
From initialization to blocking
From blocking to listening or to disabled
From listening to learning or to disabled
From learning to forwarding or to disabled
From forwarding to disabled
Bridge 2
Bridge 4
Cisco Aironet 1400 Series Wireless Bridges Software Configuration Guide
Understanding Spanning Tree Protocol
8-5
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